A block copolymer comprising a conjugated diene monomer and a vinyl aromatic monomer exhibits elasticity at room temperature even without vulcanization, the elasticity being comparable to that of vulcanized natural rubber and synthetic rubber. Further, at high temperature, the copolymer shows processability similar to that of a thermoplastic resin. Thus, the block copolymer is widely used in applications such as footwear, plastic modification, asphalt modification, an area of adhesives, household articles, packaging materials for household appliances and industrial components, toys, and the like. Furthermore, the hydrogenated product of the block copolymer has excellent weatherability and heat resistance. Therefore, in addition to the aforementioned applications, the hydrogenated product is used widely for automotive parts, medical devices, and the like.
In recent years, substitutes for a soft vinyl chloride resin have been investigated. As a substitute material having flexibility, a composition of a hydrogenated block copolymer and a polypropylene resin is disclosed (see, for example, Patent Document 1), the hydrogenated block copolymer being obtained by hydrogenating a copolymer comprising a block having a vinyl aromatic hydrocarbon and a block wherein a vinyl bond content in a diene portion is 62% or higher. The block copolymer used herein must have the high vinyl bond content in the conjugated diene portion in order to improve compatibility thereof with polypropylene. However, in case of anionic polymerization, the vinyl bond content of the diene portion is related to polymerization temperature, and in order to obtain the high vinyl bond content; the polymerization has to be conducted at low temperature. Accordingly, the productivity of the block copolymer with the high vinyl bond content has been extremely low.
As a process for producing a styrene block copolymer having such a high vinyl bond content, a method is generally employed, whereby a polar material such as tetrahydrofuran is added during anionic polymerization. However, even when such the process is employed, there was a limit to the vinyl bond content of the diene portion. Also, molecular weight distribution of the block comprising the vinyl aromatic hydrocarbon is so broad that only the block copolymer with low strength could be obtained.
Further, as a process for producing polybutadiene rubber having a high vinyl bond content suitable for tire applications, a method to polymerize 1,3-butadiene in the presence of sodium alkoxide and a polar material is disclosed (see, for example, Patent Documents 2 and 3). However, this method had a problem that a dimerization reaction occurs during polymerization, resulting in broad molecular weight distribution.    Patent Document 1: International Publication No. WO 00/15681    Patent Document 2: U.S. Pat. No. 5,654,384    Patent Document 3: U.S. Pat. No. 5,906,956